1. Field of the Invention
The present invention relates to the field of hybrid circuits of conversion and isolation between a transmission line (for example, a telephone line) and a user equipment or a collective equipment (for example, a telephone exchange or an Internet-type network provider). The present invention more specifically relates to hybrid 2 wire-4 wire circuits that are intended for enabling the sending and receiving of a wanted signal on a same transmission line, and that use a transformer. The function of such hybrid circuits is, in particular, to effect an echo cancellation function to avoid that a signal, extracted towards a receive head, be polluted by an echo coming from a signal transmitted on the line.
2. Discussion of the Related Art
FIG. 1 very schematically shows an example of application of the hybrid circuits to which the present invention applies. This drawing illustrates the connection of different equipment on a telephone line 1 and, more specifically, the connection of equipments on the subscriber side. Line 1 generally is a twin-wire line between a telephone exchange 2 and a connection element 3 on the subscriber side. To simplify, a telephone exchange 2 is considered, but it should be noted that the line transits through different collective equipment such as sub-terminal blocks, terminal blocks, etc. On the subscriber side, connection element 3 may be formed of a separator for branching the telephone line either to a telephone set 4, or to a modem 5 connected to a digital equipment 6, for example, a microcomputer. Other configurations than those described hereabove may of course exist on the subscriber side and on the collective system side. However, their detailed discussion would add nothing to the discussion of the present invention and will be accordingly omitted since it is perfectly well known.
Be it on the exchange side or on the subscriber side, the equipment are equipped with a hybrid circuit 7 used as an interface between the telephone line and the high frequency or RF signal processing circuits. Most often, telephone sets are also equipped with hybrid circuits. However, the present invention only applies to circuits intended for carrying data in bands of frequencies greater than speech frequencies.
FIG. 2 schematically shows a hybrid circuit 7 to which the present invention applies. As indicated previously, this circuit is intended for enabling the sending and the receiving of a wanted signal on a same twin-wire line 1. On the side of line 1, hybrid circuit 7 includes two inputs/outputs Rx+ and Rxxe2x88x92. On the equipment side, hybrid circuit 7 includes two inputs Tx+ and Txxe2x88x92 intended for receiving a signal to be transmitted on the line, and two outputs E+ and Exe2x88x92 intended for giving back a received signal. The inputs and outputs on the equipment side are intended for being connected to heads of transmission and reception of high frequency signals (not shown in FIG. 2), most often based, for the portion in contact with hybrid circuit 7, on low-noise amplifier circuits. Both terminals Rx+ and Rxxe2x88x92 on the line side are generally formed by the two terminals of a first winding of an isolation transformer, having the terminals of its second winding connected to an impedance matching and echo cancellation circuit enabling conversion towards the 4-wire system on the equipment side. The impedance matching portion has the function of adapting the transformer input on the equipment side with the line impedance. The echo cancellation portion has the function of suppressing, from a received signal sent to outputs E+ and Exe2x88x92, an echo coming from a transmission from terminals Tx+ and Txxe2x88x92 towards terminals Rx+ and Rxxe2x88x92. In a hybrid circuit intended for a transmission-reception of data, it is desired to obtain the smallest possible echo.
The present invention more specifically relates to hybrid circuits intended for the transmission-reception of high frequency signals in separate bands, that is, where the frequency band assigned to the transmission is different from the frequency band assigned to the reception. In such applications, the signals received% and provided by hybrid circuit 7 on terminals E+ and Exe2x88x92 are generally filtered (for example, in a high-pass filter for the subscriber side) to improve the echo attenuation.
FIG. 3 shows a conventional example of a hybrid circuit 7 applied to a transmission in separate bands. As previously indicated, the interface between line 1 and an impedance matching and echo cancellation circuit 10 is formed by a transformer 8, having a first winding 9 (arbitrarily designated as the primary winding) connected to line 1. In FIG. 3, the telephone line has been schematized by its impedance ZL across terminals Rx+ and Rxxe2x88x92 of winding 9 of transformer 8.
On the side of secondary winding 11, the impedance matching portion of circuit 10 is essentially formed of two resistors Ra, called transmission drive resistors, connecting each terminal Tx+, Txxe2x88x92 to one of terminals A, B of winding 11. Terminals Tx+ and Txxe2x88x92 correspond to differential output terminals of a high frequency transmission head 12. Generally, resistors Ra are sized according to the real part of line impedance ZL and to transformation ratio N of transformer 8. Indeed, transformer 8 most often has a transformation ratio different from one to provide an increase of the voltage level in the transmission direction from the user to the exchange. Generally, Ra=RL/2N2, where RL represents the resistance of line impedance ZL.
The echo cancellation function is effected by using the differential structure to extract, from the received signal, the echo of the transmitted signal. For this purpose, each terminal Tx+ and Txxe2x88x92 is connected to the terminal B or A opposite to that to which it is connected by resistor Ra, by means of a series association of two impedances Z1 and Z2. Output terminals E+ and Exe2x88x92 of the hybrid circuit are then formed by the respective midpoints of the series associations of impedances Z1 and Z2. Thus, terminal Tx+ is connected to terminal Exe2x88x92 by an impedance Z1 and terminal Exe2x88x92 is connected to terminal B by an impedance Z2. Terminal Txxe2x88x92 is connected to terminal E+ by an impedance Z1 and terminal E+ is connected to terminal A by an impedance Z2. The transfer function thus obtained enables that the voltage across terminals E+ and Exe2x88x92 corresponds to the voltage across terminals A and B, decreased (at least partly) by the transmission voltage across terminals Tx+ and Txxe2x88x92.
Such an echo cancellation circuit 10 operates properly if line 1 exhibits its characteristic impedance, that is, if it can be assimilated to a resistor. In this case, with Z1=2Z2, a perfect echo cancellation is obtained with purely resistive impedances.
However, the frequency of the received signal (and of the transmitted signal) varies, so that line 1 cannot be assimilated to a pure resistor. Further, the line impedance varies from one line to another, in particular according to the line length. Accordingly, the echo cancellation circuit cannot be formed of resistors only.
In conventional systems, impedances Z1 and Z2 are called compromise impedances since they are chosen according to a characteristic batch of telephone lines, generally imposed by telecommunication standards. A compromise sizing of the hybrid circuit is thus performed. The compromise impedances generally have the shapes respectively illustrated in FIGS. 4A and 4B, for impedances Z1 and Z2. Impedances Z1 are formed of a first resistor R1 in series with a capacitor C1, this in parallel with a second resistor R1xe2x80x2 (FIG. 4A). Impedances Z2 are generally formed of a resistor R2 in series with a capacitor C2 (FIG. 4B).
A first disadvantage of known circuits is that the use of compromise impedances does not allow perfect echo cancellation.
Further, in the case of a data transmission in separate bands, the spectral transmission properties result in additional difficulties due to the large width of the frequency bands transiting through the hybrid circuit. For example, for high-rate data transmission systems on asymmetrical digital subscriber lines (ADSL), that is, having a different transmission rate according to the transmission direction, the frequency spectrum used ranges from 20 kHz to 1.1 MHz.
In this example, a first frequency band from 0 to 20 kHz is dedicated to speech. A second frequency band (from 20 kHz to 1.1 MHz) intended for data transmission is itself divided in two according to the transmission direction. In a so-called xe2x80x9cupwardxe2x80x9d direction, that is, from the user to a collective equipment (for example, an Internet provider), the transmission occurs in the band from 20 kHz to 138 kHz. In the so-called xe2x80x9cdownwardxe2x80x9d direction, that is, from the collective equipment to the user equipment, the frequency band used ranges from 138 kHz to 1.1 MHz. The width difference between the upward and downward bands is linked to the desired rate difference between the two transmission directions. In such asymmetrical systems, the data rate is, for example, on the order of 500 kilobits per second in the upward direction and on the order of 1.5 to 6 megabits per second in the downward direction.
To improve the echo cancellation by means of a hybrid circuit such as illustrated in FIG. 3, a high-pass filter 13 is generally provided on the subscriber side, between output terminals E+ and Exe2x88x92 of circuit 10 and a receive head 14. The function of filter 13 is to cut-off the frequencies under the frequency band used in reception (for example, the frequencies under 138 kHz).
A disadvantage of using a high-pass filter 13 at the output of hybrid circuit 7 is that this introduces impedance matching problems between circuit 7 and filter 13.
It should be noted that a digital filtering solution cannot be provided due to the large bandwidth, combined with the resolution of the analog-to-digital conversion that is necessary to be obtained due to the mixing of the transmitted and received signals and to the relative high level of the transmitted signal.
It should further be noted that, for reasons of noise and dynamics, filter 13 is generally formed by means of discrete inductive and capacitive components.
Another problem that is raised in the context of high-rate asymmetrical transmissions is that the signal to be transmitted is of high level while the received signal has a very low level (especially for long lines, for example of more than 4-5 km, resulting in a strong attenuation). In the transmission direction, the voltage increase effected by the transformer is compatible with the needs of high-rate transmissions that require a high transmission power to overcome the noise levels. However, in the reception direction, the attenuation effected by the line transformer adversely affects the system operation. Indeed, the signal receiver must then have, at its input, a noise source of very low level, which makes its implementation difficult. The noise voltage, at the receiver input, must be all the smaller as the voltage is low across the transformer (the winding located on the equipment side).
The document EP-A-0 678 979 discloses an hybrid circuit of the same type as the present invention. This circuit comprises means for separating frequency bands transmitting data (40 kHz-1.7 MHz) from the vocal bands ( less than 4 kHz). Echo-cancelling means are provided and are distinct from impedance circuits associated with line transformer. A separation between the transmission and reception bands is provided downstream from the transmission/reception heads with respect to the line.
According to a first aspect, the present invention aims at providing a novel solution for suppressing the echo in a separate band transmission system that overcomes at least one disadvantage of known solutions.
The present invention aims, more particularly, at providing a novel hybrid circuit for high-rate separate band data transmission.
The present invention also aims at providing a solution that suppresses the use of compromise impedances.
According to a second aspect, the present invention further aims at making less critical the reception noise problem and, in particular, at enabling, with respect to a conventional transformer, an increase of the level of the received signal without adversely affecting the increase of the transmitted signal effected by the line transformer.
To achieve these objects, the present invention provides a hybrid circuit forming an interface between a transmission line and heads of transmission-reception of high frequency signals in bands of different frequencies in transmission and reception, including a line transformer, and means for separating transmission/reception bands combined with echo cancellation means.
According to an embodiment of the present invention, the band separating means are formed of a filter having two first input impedances directly connected, respectively, to two end terminals of a first winding of the transformer.
According to an embodiment of the present invention, two second input impedances of the filter are directly connected, respectively, to two input terminals of the circuit, intended for receiving a signal to be transmitted.
According to an embodiment of the present invention, the sizing of the filter and of the echo cancellation means components is independent from the impedance of the line and from the transformation ratio of the transformer.
According to an embodiment of the present invention, the filter is of third order.
According to an embodiment of the present invention, the circuit includes, in series between each end terminal of the first winding of the transformer and one of said two input terminals of the circuit, a first and a second input impedance of the filter, the midpoints of these series associations being connected to each other by an LC cell, and each midpoint being connected, via a capacitive or inductive element of an output impedance of the filter, directly to one of two output terminals of the circuit that are interconnected by a resistor common to both output impedances.
According to an embodiment of the present invention, each input impedance includes a series association of a resistor with a parallel assembly of a first capacitor and of an inductance, the respective midpoints of the series associations of the two first and the two second input impedances being connected to each other by a second capacitor, and said associations belonging to each first input impedance being connected in series with one of said associations belonging to the two second input impedances, the midpoints of these series connections being directly connected to one of two output terminals of the circuit that are connected to each other by a resistor.
According to an embodiment of the present invention, each input terminal of the circuit is connected, by a drive resistor depending on the impedance of the line and on the transformation ratio of the transformer in transmission, to a terminal of the first winding of the transformer.
According to an embodiment of the present invention, the band separating means are formed of a high-pass filter.
According to an embodiment of the present invention, the band separating means are formed of a low-pass filter.
According to an embodiment of the present invention, the first winding of the transformer is formed of at least three series windings, the respective numbers of spirals of which are a function of the desired transformation ratios in transmission and reception, a transmission signal being applied across the terminals of a central winding while a reception signal is sampled across the end terminals of the series association of the windings.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.